Page 196 - Tribology in Machine Design

Sliding-element bearings 181

This is a high rate for a small bearing. It can be decreased by using a smaller
recess or lower pressure, together with a larger outside diameter, or by using
a more viscous oil. Also smoother and squarer surfaces will reduce the film
thickness requirement, which is halved to 0.025 mm and would decrease Q
to one-eighth.
The input power to the pump is, by eqn (5.15)

The rotational speed is ri = 1740/60= 29 r.p.s., and the torque required to
rotate the bearing is, by eqn (5.16)

The power required to rotate the bearing is

The mean radius of the section of the pad where the film shear is high is

At this radius, we may imagine a tangential, concentrated friction force,

The coefficient of friction is the tangential force divided by the normal force,
or

If lubrication were indifferently provided, with no recess, and the coefficient
of friction/were 0.05 at a radius r f = 17 mm, the power requirement would
be

This is eight times the power lost altogether at the bearing and pump in the
externally pressurized bearing.

5.3. Squeeze-film Bearings which are subjected to dynamic loads experience constantly
lubrication bearings changing thicknesses of the oil film. Also, as a result of fluctuating loads, the
lubricant is alternately squeezed out and drawn back into the bearing.
Together with the oil supplied through correctly located grooves, a
parabolic velocity profile with changing slope is obtained. This is illustrated
in Fig. 5.7. The load-carrying ability, in such cases, is developed without the
sliding motion of the film surfaces. The higher the velocity, the greater is the